@	Special Issue: Recent progress in basic studies related to hydrocarbon deposits (Part ‡T)
		Miocene counterclockwise rotation of Northeast Japan: a review and new model
		Late Miocene to Pliocene volcanism and reservoir formation in the Akita-Yamagata Oil Field, northeast Japan
		Sedimentological and mathematical analysis of the Neogene turbidite succession along the Gomoto River through the Western oil belt in the Niigata Neogene sedimentary basin, central Japan
		Diatom biostratigraphy of the Miocene sequence in the southern Kanazawa area, Ishikawa Prefecture, central Japan
		Diatom biostratigraphy of the Miocene sequence in the Iozen area, Hokuriku Province, central Japan

Miocene counterclockwise rotation of Northeast Japan: a review and new model

Hiroyuki Hoshi (Department of Environmental Earth Science, Aichi University of Education) and Masaki Takahashi (Mineral and Fuel Resources Department, GSJ)

1999

vol. 50 (1) P. 3-16

10 figs., 1 table

Keywords: Japan Sea opening, Miocene, Northeast Japan, paleomagnetism, rotation, tectonics

Abstract: The paleomagnetic studies which have been performed in the forearc side of NE Japan during the last decade are reviewed in detail to elucidate the Miocene rotational process of NE Japan. The areas which are dealt with in this review are as follows; from north to south, Ninohe, Miyako and Kamaishi Mine in Iwate Prefecture, Shiogama-Matsushima in Miyagi Prefecture, Ryozen in Fukushima Prefecture, and Motegi over a Tochigi-Ibaraki prefectural boundary. We have compiled reliable paleomagnetic directional data from the areas and established variations in declination and inclination with age. In an age-declination diagram, a pronounced change can be seen at about 20 Ma; declination seems to have varied from ca. 295‹at 21 Ma to 0‹at 18 Ma. All the data showing large counterclockwise deflection in declination, 60‹or more, were taken from the lower Miocene and older formations in the Kitakami Mountains. Two models can account for the observed paleomagnetic rotation. One is a large counterclockwise crustal rotation of NE Japan. This model requires more than 60‹of counterclockwise rotation of entire NE Japan. The amount of rotation, however, is much larger than the angle of rotation expected from the reconstruction based on geology and physiography. Another model, which we consider more plausible, explains that differential rotation occurred in NE Japan. This model interprets that the angle of rotation of the Kitakami Mountains was larger than that of the Abukuma Mountains. Dextral displacement on a fault between the Kitakami and Abukuma Mountains, which has been presumed and termed the Chokai-Ishinomaki Tectonic Line, may have accommodated the larger rotation of the Kitakami Mountains.

Late Miocene to Pliocene volcanism and reservoir formation in the Akita-Yamagata Oil Field, northeast Japan

Nobuyuki Tsuchiya (Geology Department, GSJ)

1999

vol. 50 (1) P. 17-25

3 figs., 2 tables

Keywords: Akita, Yamagata, oil field, volcanic rock, Miocene, Pliocene, cluster, reservoir rock, northeast Japan

Abstract: Late middle Miocene to Pliocene andesite-dacite volcanism took place and formed many submarine volcanoes in the Akita-Yamagata oil field basin (Aosawa rift), in the northeast Japan arc. Most of the volcanoes are distributed in four areas, and formed clusters in the basin. These areas are the Subari-Nanakura, Manaitayama, Suwayama-Yonegamori, and Momoyake-Aosawa areas. Each area is 20-30km in diamiter and includes 8 to 15 submarine volcanic bodies, and some of them reserve hydrocarbon deposits. As the volcanic rocks in the oil field basin are rich in fracture and vesicle, the rocks could reserve large volume of oil and gas.
@Most of the volcanic rocks reservoirs are 1) basalt vesiculous lava and hyaloclastaite of middle Miocene, 2) lava and volcaniclastic rock of andesite to dacite of late Miocene, and 3) their reworked sediments. The volcanoes in the Subari-Nanakura, Manaitayama, and Momoyake-Aosawa areas include mainly andesite volcanoes, and small amount of rhyolite-dacite. The volcanism in the three areas continued from late Miocene to early Pliocene time. However, the Suwayama-Yonegamori area situated in the eastern part of the oil field basin, and includes mainly dacite-rhyolite volcanoes which formed many lava domes of late middle Miocene to late Miocene. As few volcanic rocks are dated at late middle Miocene, the volcanism in this time may have been weakened in the oil field. Also, the volcanism in the oil field may have been practically interrupted during late Pliocene to early Pleistcene time, because no volcanic rock dated in this time.

Sedimentological and mathematical analysis of the Neogene turbidite succession along the Gomoto River through the Western oil belt in the Niigata Neogene sedimentary basin, central Japan

Shuichi Tokuhashi (Mineral and Fuel Resources Department, GSJ), Yoshiro Ishihara (Faculty of Science, Yamaguchi University) and Yuichiro Miyata (Faculty of Science, Yamaguchi University)

1999

vol. 50 (1) P. 27-48

12 figs., 1 table

Keywords: central Japan, Niigata oil basin, Nishiyama oil belt, Gomoto River, Neogene, turbidite, sedimentary facies analysis, time series analysis, channel-levee system, Milankovitch cycles

Abstract: Sedimentological and mathematical analysis of the turbidite succession along the Gomoto River through the Western Oil Belt in the Niigata Neogene Backarc Basin revealed the following results:
@1) The frequent intercalation of CCC-turbidite sandstones (Walker,1992) as well as debris flow deposits in the succession and the stable paleocurrent direction from south to north measured from the foreset laminae of the current climbing ripples strongly suggest that the turbidites in the succession were deposited as overbank deposits along the main channel which stretched from the south to the study area in the basin.
@2) The periodicity of the cyclic variation recognized in the number and total thickness of the turbidite sandstone beds intercalated in the specific thickness of the hemipelagic mudstone, i.e. 100,000 and more years, may reflect the Milankovitch cycles.

Diatom biostratigraphy of the Miocene sequence in the southern Kanazawa area, Ishikawa Prefecture, central Japan

Yukio YANAGISAWA (Geological Museum, GSJ)

1999

vol. 50 (1) P. 49-65

9 figs., 2 tables

Keywords: diatom, sedimantary rocks, glauconite, biostratigraphy, biochronology, magnetostratigraphy, Kanazawa, Ishikawa Prefecture, Hokuriku, Japan, Neogene, Miocene

Abstract: Diatom biostratigraphy is established for the middle Miocene sequence in the southern Kanazawa area, Hokuriku Province, central Japan. Diatom zones of the upper Crucidenticula kanayae Zone (NPD 3A), the Denticulopsis praelauta Zone (NPD 3B) through the middle D. lauta Zone (NPD 4A) are identified with a number of stratigraphically useful diatom biohorizons. The Miocene sequence in this area is composed of the Iozen, Sunagozaka, Nanamagari, Asagaya, Shimoaraya and Saikawa Formations in ascending order. The upper Nanamagari Formation is assigned to the upper C. kanayae Zone (NPD 3A). The Denticulopsis praelauta Zone (NPD 3B)is represented by a two-meter thick glauconite bed at the base of the Asagaya Formation. The Asagaya and Saikawa Formations are correlated to the lower D. lauta Zone (NPD 4A) below the last occurrence (LO) of Cavitatus lanceolatus. Useful diatom biohorizons including the first occurrence (FO) of D. praelauta (D35, 16.3 Ma), the FO of D. lauta (D40, 15.9 Ma) and the LO of D. praelauta (D41, 15.7 Ma) are recognized in the lowermost Asagaya Formation, and the FO of C. lanceolatus (D41.5, 15.6 Ma), the FO of Denticulopsis okunoi (D42, 15.5 Ma) and the LO of D. okunoi (D43, 15.4 Ma) are identified in the Saikawa Formation. Correlation of the diatom biostratigraphy to the magnetostratigraphy of Itoh and Ito (1989) is not concordant with Barron and Gladenkov' (1995) magnetobiostratigraphic framework. The glauconite bed at the base of the Asagaya Formation shows a very low sedimentation rate from 16.4 to 15.9 Ma. Biostratigraphic age for the top of the Saikawa Formation is discordant between diatom and calcareous nannofossil biostratigraphies, and thus further detailed lith- and biostratigraphic studies for this formation is requires.

Diatom biostratigraphy of the Miocene sequence in the Iozen area, Hokuriku Province, central Japan

Yukio YANAGISAWA (Geological Museum, GSJ)

1999

vol. 50 (1), p. 67-81.

9 figs., 2 tables

Keywords: diatom, sedimantary rocks, biostratigraphy, biochronology, Iozen, Ishikawa Prefecture, Toyama Prefecture, Hokuriku, Japan, Neogene, Miocene

Abstract: Diatom biostratigraphy is established for the Middle Miocene sequence in the Iozen area, Hokuriku Province, central Japan, and the high-resolution Neogene diatom biochronology proposed by Yanagisawa and Akiba (1998) was proven to be applied to the Middle Miocene formations in this area. The Miocene sequence in this area is composed of the Iozen, Sunagozaka, Doyama, Omine, Kurahara and Takakubo Formations in ascending order. The IV/V members of the Doyama Formation, Omine and the lower Kurahara Formations are assigned to the Denticulopsis lauta Zone (NPD 4A). The FO (first occurrence) of Cavitatus lanceolatus (D41.5, 15.6 Ma) is in the uppermost Doyama Formation. The FO of Denticulopsis okunoi (D42, 15.5 Ma) and the LO of D. okunoi (D43, 15.4 Ma) are found in the Omine Formation, and the LO of C. lanceolatus (D43.2, 15.2-15.3 Ma) are identified in the lower Kurahara Formation. Detailed diatom biostratigraphy of this study clearly indicates that a previously proposed Miocene lithostratigraphic correlation between the Iozen and Kanazawa areas is partly erroneous. The Omine Formation, which has been treated as an equivalent of the Asagaya Formation in Kanazawa area, should be correlated with the overlying Saikawa Formation. The IV/V and III members of the Doyama Formation are correlated with the Shimoaraya and Asagaya Formations, respectively.